In a secondary battery employing a lithium anode it is desirable to employ an electrolyte system which provides high cycling efficiency, good conductivity, and reasonable cost. The number of times a lithium battery can be recharged, and the efficiency of each recharging are the measures of cycling efficiency. Cycling efficiency of the Li electrode is primarily a function of the ability of the electrolyte solvent to withstand reduction by lithium, which is a powerful reducing agent. When reduction occurs, reaction products form on the surface of the lithium electrode preventing subsequent smooth replating during the recharge cycle. This results in dendritic growth and consequently poor lithium morphology. Poor lithium morphology can also result from poor throwing power of the electrolyte or from specific electrolyte adsorption effects. Poor lithium morphology can result in: (1) electrical isolation of some of the plated lithium, making it unavailable for stripping during discharge; (2) short circuits between the electrodes due to dendritic growth; and (3) a rapid chemical reduction rate due to the increased electrode surface area. Also important is the ability of a solvent to dissolve the amount of salt necessary to provide adequate conductivity, and the conductivity features of particular salt and solvent choices.
A number of prior batteries--both primary and secondary--have employed a mixture of two or more solvents in an electrolyte system. A mixed solvent is disclosed in Garth U.S. Pat. No. 3,778,310, duPont British Pat. No. 1,343,853, Eisenberg et al. U.S. Pat. No. 3,468,716, Maricle et al. U.S. Pat. No. 3,567,515, Maricle et al. U.S. Pat. No. 3,578,500, Hovsepian U.S. Pat. No. 3,877,983, Klemann et al. U.S. Pat. No. 4,060,674, and Mayer et al. U.S. Pat. No. 3,185,590.
Although diethylether (DEE) is mentioned as a possible solvent component in a number of the above patents, and is even used as the sole solvent of one example in the Mayer patent, a problem associated with DEE is its very low conductivity. While conductivity can be improved by mixing DEE with a more conductive cosolvent, in general in a secondary cell one would expect a corresponding sacrifice of cycling efficiency, since the more conductive cosolvent choices are also more lithium-reactive.
Many possible solvents and combinations of solvents are mentioned in the above patents, including unsaturated heterocycles and saturated ethers such as DEE and tetrahydrofuran (Garth and duPont); pentacyclic esters, aliphatic ethers such as DEE and tetrahydrofuran, cyclic ketones, and aliphatic nitriles (Eisenberg); sulfur dioxide, trialkyl borates, boronic acid esters, tetraalkyl silicates, nitro alkanes, lactams, ketals, orthoesters, monoethers such as DEE, cyclic ethers such as tetrahydrofuran, dialkl sulfates, and alkyl sulfonates (Maricle); tetrahydrofuran, dimethyl carbonate, propylene carbonate, 1, 2 dimethoxyethane, dimethylformamide, trimethyl carbonate, ethyl-N, N-dimethyl carbonate, the dimethyl ether of diethylene glycol, cyclic ethers such as 1, 3 dioxolane, 4-methyl, 1, 3 -dioxolane, ethylene oxide, propylene oxide, butylene oxide, dioxane, and tetrahydrofuran, and aliphatic ethers such as 1, 2-dimethoxyethane, the dimethyl ether of diethylene glycol, and the diethyl ether of diethylene glycol (Hovsepian); ethers, esters, sulfones, sulfites, nitrites, and nitrates (Klemann); and ethers such as DEE, amines, amides, sulfoxides, and nitriles (Mayer).